1. Field of the Invention
This invention relates to poly-2-pyrrolidone compositions containing a small amount of an organic isocyanate and/or a thermal precursor thereof. In another aspect, this invention relates to methods of melt extruding, and especially melt spinning, such compositions.
2. The Prior Art
Poly-2-pyrrolidone is produced by the alkaline-catalyzed polymerization of 2-pyrrolidone in the presence of carbon dioxide (see U.S. Pat. No. 3,721,652). Polypyrrolidone so-produced can be melt-spun into filaments by extrusion from multi-hole spinnerets. In melt-spinning, the polymer composition is extruded in a molten condition at a melt temperature which is generally greater than about 270.degree. C. This extrusion must be carried out with care because of the propensity of the polymer to thermally degrade, reverting back to monomer. As well as causing substantial product loss and process ineconomies this can also cause bubbles and the formation of voids or pox marks in the extrudate or filaments. Moreover, in addition to monomer reversion, the molecular weight of the resulting poly-2-pyrrolidone filaments is less than the molecular weight of the original polymer, and the filaments have lower tensile strength and poorer fibrillation properties.
If extrusion is attempted at appreciably lower temperatures to avoid thermal decomposition, the material is not properly melted and fibers of substantially lower tensile strength are produced. Consequently, in order to melt extrude polypyrrolidone efficiently, one may either seek to increase the thermal stability of the polymer, or to improve the extrudability of the polymeric composition.
The unusual propensity of poly-2-pyrrolidone to thermally degrade back to its monomer and the adverse effect on melt spinning or extrusion is recognized to be a serious problem and the prior art has made a number of attempts to mitigate this problem. For example, U.S. Pat. No. 3,052,654 attempts to reduce degradation occurring during melt extrusion by pre-heating the poly-2-pyrrolidone to remove degradation products, e.g., pyrrolidone. U.S. Pat. No. 3,017,393 teaches that the extrusion degradation problem may be alleviated by washing or treating the polymer with an aqueous organic carboxylic acid solution and similarly, U.S. Pat. No. 3,072,615 teaches using dilute aqueous fatty acid.
U.S. Pat. No. 3,009,893 teaches that the problem may be reduced by the incorporation of relatively large amounts of certain substances which apparently function as melting point depressants.
Japanese Pat. No. 44-23509 (1969) discloses a two step process for improving the thermal stability of polypyrrolidone which comprises first treating polypyrrolidone with an alcohol in the presence of a strong inorganic acid or organic sulfonic acid followed by treatment with a monoisocyanate. Patentee further teaches that if the monoisocyanate treatment is performed without the prior alcohol treatment the polypyrrolidone will be colored and its thermal stability will be greatly reduced.
Japanese Pat. No. 43-13059 (1968) discloses a two step process for improving the thermal stability of polypyrrolidone which comprises treating polypyrrolidone with an aqueous boric acid solution followed by treatment with an organic isocyanate. Patentee also teaches that if the monoisocyanate treatment is performed without the prior alcohol treatment the polypyrrolidone will be colored and its thermal stability will be greatly reduced.
Also, with respect to nylon-6 the text Nylon Plastics, edit by Melvin I. Kohan; John Wiley & Sons teaches on page 67, Table 2-3 that the salt formation or reaction of nylon-6 with isocyanate eliminates end groups to decrease the rate of lactam monomer formation in nylon-6.
The prior art has also proposed various other additives to mitigate the thermal monomer reversion of nylon-4, however, the problem is necessarily empirical in nature and until a given substance has actually been tried, it cannot be predicted whether it will have a beneficial effect.